This research proposal has combined biochemical and cellular investigation on the fidelity of DNA synthesis. The objective is to determine the cellular mechanism that guarantees the accurate copying of the nucleotide sequence of DNA during each cell's division. We also intend to determine whether deficiencies in the accuracy of DNA synthesis are causally associated with mutations and malignancy. Our specific objectives are: 1) to define the accuracy by which DNA polymerases and DNA replicating complexes copy natural DNA in vitro; 2) to determine if particular nucleotide sequences in DNA are copied with more frequent mistakes than are others; 3) to study the effect of mutagens and carcinogens on the nature and frequency of mis-incorporation by DNA polymerases; 4) to search for specific endonucleases in cells that can excise erroneous nucleotide incorporations made during DNA replication or repair; 5) to analyze the molecular mechanism by which DNA synthesis occurs opposite thymidine dimers and to determine the specificity for nucleotide insertion at these sites; 6) to investigate the relationship between diminution in the fidelity of DNA synthesis and tumor progression.